As a result of defense mechanism and as by-products of cellular metabolic processes which utilizing oxygen in human body, certain molecules called reactive oxygen species (ROS) and reactive nitrogen species (RNS) are normally produced. Later these molecules are converted into free radicals in the body. Many factors are associated with the production of free radicals viz. smoking, drinking alcohol, high fat diet, too much sun exposure, existence of pollutants in the air and even excessive much exercise and stress. Exogenous sources of ROS include asbestos, crystalline silica, coal, diesel, chromium, bleomycin, herbicides, nitric oxide, ozone, radiation, cigarette smoke. Free-radical mechanisms may cause several human diseases including cancer, atherosclerosis, malaria, rheumatoid arthritis and neurodegenerative diseases. Superoxide radical (O2) and hydrogen peroxide (H2O2) are known to be generated in the brain and nervous system in vivo. When these free radicals are produced in excess, they can cause tissue injury. However, tissue injury can itself cause ROS generation, which may contribute to a worsening of the injury. ROS are involved in cell signaling, gene transcription, aging and enzymology. A variety of cancers generate ROS as well as diabetes and pulmonary conditions ROS generation is also associated with oxidation of lipoproteins.